Modern vehicle fuel systems vent the fuel tank to a vapor storage canister to recover the fuel vapors that naturally and continually form, often called the diurnal vapors. The stored vapors are later purged and burned in the engine. The tank pressure is thus kept fairly constant without venting the vapors directly to atmosphere. The diurnal vent is generally quite small and restricted, so as not to encourage diurnal vapor formation. When the tank is filled, a much greater volume of fuel vapor is displaced from the tank by the entering liquid fuel, and in a relatively short time. While the refilling event occurs infrequently, future emission standards may require that displaced fuel fill vapors be recovered as well.
As a consequence, many designs have been proposed for a vapor vent valve that will allow the tank to vent quickly, across a large sized vent opening, but only when the tank is being filled. Normally, the large size vent opening is closed, to discourage excessive diurnal venting. Most proposed designs locate the vapor vent valve on or near the end of the filler neck, so as to take advantage of mechanical motions that occur only during or just before the fuel filling event to open and close the valve. For example, removal and replacement of the filler neck closure cap, or insertion and removal of the filler nozzle, may be used to open and close the valve. One disadvantage of such a design is that a good deal of volume is occupied at the end of the filler neck, where space may be tight. Another is that a vapor vent line must be provided from the tank over to the valve, and then from the valve back over to the canister. In order to avoid such a lengthy vapor exit path, other designs propose to mount the vapor vent valve directly to the top wall of the tank. If it is also desired to actuate such a valve by the same kinds of mechanical motions, then some physical connection, like a cable or wire, must be run from the end of the filler neck to the tank mounted valve. With some vehicle body designs, it may be difficult to establish or service that kind of remote connection.